The invention relates to rotary engines of the type disclosed in U.S. Pat. No. 2,988,065 granted on June 13, 1961 to Wankel et al. Such rotary combustion engines are available commercially for operation on the Otto cycle with spark ignition with compression ratios of approximately 8:1 to 10:1. In addition, fuel injection versions of such spark-ignited rotary engines have been designed, for example, as shown in U.S. Pat. No. 3,246,636 granted Apr. 19, 1966 to Bentele. For diesel-type operation, higher compression ratios are required, for example, a compression rate of approximately 15:1 or higher. U.S. Pat. No. 3,331,358 granted July 18, 1967 to Muller et al is an example of a diesel rotary engine of this type.
At this point it should be noted that by compression ratio of an internal combustion engine is meant the ratio of the maximum to the minimum volume of a working chamber of the engine. Thus, an 8.5:1 compression ratio means that the maximum volume of a working chamber is 8.5 times the minimum volume. In a four-stroke cycle internal combustion engine having an 8.5:1 compression ratio, and taking in air at atmospheric pressure at a temperature of 70.degree. F, the actual pressure in a working chamber, as a result of compression, may reach about 20 atmospheres and a temperature of about 760.degree. F. It is this increase in temperature of the working fluid that causes the actual pressure to rise to about 20 atmospheres rather than, if there were no temperature rise, to only 8.5 atmospheres. Accordingly, for reasons of clarity, what is usually referred to as simply the compression ratio is herein termed the volumetric compression ratio.
As disclosed in said Wankel et al patent, the rotary engine has a multi-lobe cavity which preferably has basically an epitrochoidal profile. The shape of the epitrochoidal engine cavity determines the volumetric compression ratio. Thus, an epitrochoid having a smaller ratio of a/b has a larger volumetric compression ratio, where a is equal to one-half the length of the major axis of the epitrochoid and b is equal to one-half the length of its minor axis. Today it is more common to express the shape of the epitrochoid in terms of a so-called "K" factor which is equal to the ratio R/e where R is the radial distance from the center of the rotor to the tip of its apex seals and e is the distance between the rotor center and the engine axis. In general, the magnitude of the K factor increases as the ratio a/b decreases. Hence, for higher compression, a rotary engine of the type shown in said Wankel et al patent should have a high "K" factor.
As is evident from the disclosure of said Wankel et al patent, at high volumetric compression ratios the shape of each engine working chamber at its top dead center position becomes extremely thin in a radial direction and therefore combustion in the working chambers is subject to severe cooling or chilling by the radial walls of the chamber. For this reason, it is difficult to make a successful diesel-type rotary combustion engine of the type disclosed in the Wankel et al patent simply by changing the engine "K" factor to increase the engine volumetric compression ratio.
Diesel-type rotary combustion engines have been designed, for example, by using a supercharger as in U.S. Pat. No. 3,858,557 granted on Jan. 7, 1975 to Myers et al. Such diesel-type engines, however, require the added complexity of a supercharger. Diesel-type rotary combustion engines have also been designed using a generally square rotor mounted within a three-lobed epitrochoid with the porting designed to provide for double compression strokes. This latter type of rotary diesel is shown in U.S. Pat. No. 3,097,632 granted on July 16, 1963 to Froede et al. This latter configuration results in a substantially larger and heavier engine for a given power output than is obtainable with an engine configuration employing a generally triangular rotor mounted within a two-lobed epitrochoid.